1. Field of the Invention
This invention relates to a pressure-sensitive recording sheet and a coating material therefor.
2. Prior Art
A pressure-sensitive recording sheet is referred to as non-carbon paper. Generally, pressure-sensitive recording sheets are composed of a top (CB: Coated Back) sheet, a middle (CFB: Coated Front and Back) sheet and a bottom (CF: Coated Front) sheet. The back surface of each of the top sheet and the middle sheet is coated with microcapsules containing a solution of leuco dye as a core substance.
The upper surface of each of the middle sheet and the bottom sheet is coated with a layer containing a color-developing agent composed of an acidic substance. They form a color upon the application of a writing, typewriter, mechanical pressure, thus permitting duplication of several copies.
The pressure-sensitive recording sheets have as a defect an unwanted color-forming at the occasion when a recording is unnecessary, which is undesirable for copying. Particularly, it is known that the hard readability (background-smudge) of letters which decreases the value of a pressure-sensitive recording sheet (as background-smudge) is caused by an unwanted color-forming under a severe thermal environment which occurs during the room- or warehouse-storage, the transport, or the press work for producing an account book. As preventive methods of an unwanted color-forming (i.e. the background-smudge) under thermal environments, there have been proposed a use of thick capsule wall, a method for protecting microcapsules by means of the addition of water soluble polymer or latex to the coating material containing microcapsules, and the like. However, the thick capsule wall requires a large amount of wall material, which increases the capsule-production costs. The addition of water-soluble polymer or latex increases the capsule-production costs, and further the excess addition reduces a color-forming property and has a bad effect on the function of a pressure-sensitive recording sheet, which is undesirable for the recording sheet.
The techniques similar to those of this invention are disclosed in Japanese Patent Laid-Open Application No. 62-267184 and in Japanese Patent Publication No. 53-21328. Japanese Patent Laid-Open Application No. 62-267184 discloses a pressure-sensitive recording sheet in which a substrate is coated with a mixture of microcapsules (1) containing a solvent which dissolves a colorless dye for a pressure-sensitive recording sheet, and of microcapsules (2) containing a dye-free solvent and having larger particles than microcapsules (1). The object of Japanese Patent Laid-Open Application No. 53-21328 consists in a counterplan against the smudge of sheet in the flow-out of the recording solvent by the rupture of microcapsules, that is, the counterplan of the smudge through the rupture of dye-free microcapsules (2) by rubbing, accidentally slight pressure, etc.
The solvent in microcapsules (2) having large particles is independent of the solvent in microcapsules containing dye, and it is an odorless solvent having high boiling point without preventing the color-development of a leuco dye.
In this case, it seems that the solvent in microcapsules (2) is the same as the solvent in microcapsules (1).
Japanese Patent Publication No. 53-21328 describes that microcapsules (1) contain a solution prepared by dissolving a colorless dye in a solvent having a weak affinity to a color-developing agent, and that microcapsules (2) contains a solvent having a strong affinity to a color-developing agent. The solvent having a strong affinity to a color-developing agent is the same as a solvent used in microcapsules A of this invention. The object of Japanese Patent Publication No. 53-21328 consists in the prevention of the smudge without the color-development by the rupture of one of two kinds of microcapsules through an outer pressure.
Each of the above Application and Publication does not describes about the thermal smudge, and the use of technologies in these Application and Publication as a counterplan against thermal smudge is insufficient or ineffective.